Waste Incineration Heat and Seasonal Thermal Energy Storage for Promoting Economically Optimal Net-Zero Energy Districts in Finland

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Article Waste Incineration Heat and Seasonal Thermal Energy Storage for Promoting Economically Optimal Net-Zero Energy Districts in Finland

Janne Hirvonen 1,* and Risto Kosonen 1,2

1 Department of Mechanical Engineering, Aalto University, 00076 Aalto, Finland; risto.kosonen@aalto.ﬁ 2 College of Urban Construction, Nanjing Tech University, Nanjing 210000, China * Correspondence: janne.p.hirvonen@aalto.ﬁ; Tel.: +358-50-431-5780

Received: 15 October 2020; Accepted: 11 November 2020; Published: 17 November 2020

Abstract: In countries with high heating demand, waste heat from industrial processes should be carefully utilized in buildings. Finland already has an extensive district heating grid and large amounts of combined heat and power generation. However, despite the average climate, there is little use for excess heat in summer. Waste incineration plants need to be running regardless of weather, so long-term storage of heat requires consideration. However, no seasonal energy storage systems are currently in operation in connection with Finnish waste incineration plants. This study used dynamic energy simulation performed with the TRNSYS 17 software to analyze the case of utilizing excess heat from waste incineration to supplement conventional district heating of a new residential area. Seasonal energy storage was utilized through a borehole thermal energy storage (BTES) system. Parametric runs using 36 different storage configurations were performed to find out the cost and performance range of such plans. Annual energy storage efficiencies from 48% to 69% were obtained for the BTES. Waste heat could generate 37–89% of the annual heat demand. Cost estimations of waste heat storage using BTES are not available in the literature. As an important finding in this study, a levelized cost of heat of 10.5–23.5 €/MWh was obtained for various BTES configurations used for incineration waste heat storage. In the three most effective cases, the stored heat reduced annual CO2 emissions of the residential area by 42%, 64% and 86%. Thus, the solution shows great potential for reducing carbon emissions of district heating in grids connected to waste incineration plants.

Keywords: seasonal thermal energy storage; waste incineration; district heating; waste heat

1. Introduction In the heating dominated climate of Nordic countries, heating demand surpasses electricity demand. In Finland, Sweden and Denmark, the combined heating demand is about 400 TWh and is mostly met through district heating [1]. However, district heating is typically produced through combustion of fossil fuels, biomass or solid waste, all of which produce carbon emissions. Thus, there is great potential for reducing carbon emissions in the district heating sector. The reductions could be realized through improved system coupling of waste incineration plants and district heating. Currently, there are many waste incineration plants in Sweden and Finland, which are used for generating electricity and heat. In Finland, 93% of non-sorted mixed waste is utilized for energy generation [2]. The energy sector in Finland is dominated by co-generation plants. Unfortunately, while solid waste is produced all through the year, the demand for heat falls during summer and part of the heat produced by incineration is wasted. In addition, due to the already high energy conversion of waste, energy generated from waste cannot really be increased. Thus, the available energy should be used more eﬀectively.

Buildings 2020, 10, 205; doi:10.3390/buildings10110205 www.mdpi.com/journal/buildings Buildings 2020, 10, 205 2 of 19

One solution is seasonal underground thermal energy storage. This could be done using aquifer thermal energy storage (ATES), where heat is moved between a hot and cold water reservoir. It has been successfully utilized in the Swedish Arlanda airport, where thermal energy storage has been reported to supply 22 GWh/a of cooling and low temperature heat. In a Finnish study, district cooling was generated using heat pumps and waste heat was stored in the hot reservoir of an ATES system. Heat was produced at the cost of 41.5 €/MWh, while the seasonal storage efficiency was 73–83%. Unfortunately, appropriate aquifers are not available everywhere. Thus, a more general solution could be obtained by using borehole thermal energy storage (BTES) technology. This means storing heat into the ground itself using boreholes fitted with heat transfer pipes. In many studies, the heat to be stored comes from solar energy [3]. Finnish studies on seasonal solar energy storage have shown that it is technically feasible, but economically problematic. The most expensive components of the system are the solar thermal collectors [4] or the solar photovoltaic panels [5], not the actual storage system. Thus, a system with a free source of heat would be a potential breakthrough. In Emmaboda, Sweden, a high temperature BTES system was built to store waste heat from industrial processes, such as metalworking [6]. The storage volume had a high thermal conductivity of 3.0 W/mK. There were 140 boreholes, each 150 m deep and equipped with a coaxial double tube borehole heat exchanger (BHE). Seasonal storage efficiency of 68% was expected. However, technical problems prevented heat extraction during the initial test period. Heat was first extracted after the fourth year of operation and design performance was expected to be achieved after the fifth year [7]. Further study of the facility revealed that the industrial waste heat was of lower temperature than expected, which reduced the output from the BTES system [8]. Only 19% storage efficiency was achieved. Total construction and consultancy cost of €1.25 M was reported, but not the cost of produced heat. Other examples on storage of industrial waste heat are scarce. A 50,000 m3 BTES located in China was designed to utilize both industrial waste heat and solar energy [9]. The supply temperature in the system varied according to the specific process that was supplying the heat at any given moment. However, the maximum temperature for the processes varied from 60 ◦C to 90 ◦C. The storage volume had a low thermal conductivity of only 1.1 W/mK. The ground in the core of the BTES was heated up to 40 ◦C, though this was expected to rise with continued operation. This was purely a technical study with no cost information reported. One experimental BTES system was connected to a combined heat and power plant in the Czech Republic [10]. The core temperature of the facility reached up to 78.5 ◦C. This temperature was measured in a dedicated monitoring borehole, so as not to be disturbed by operational heat flows. The average measured thermal conductivity in the ground was a moderate 2.2 W/mK. It was estimated that 65% of injected heat could be sustainably extracted. This is promising for systems intending to utilize heat from waste incineration plants. However, no estimation of the cost of heat was presented. The main goal of the present study was to show the technical and economic feasibility of borehole thermal energy storage as a supplementary heating system in Finnish district heating grids, where large enough waste heat streams are available. The study also aimed to explain why certain solutions are more cost-effective than others, taking into account the cold Finnish climate and the local ground properties. The novelty of the paper was in the integration of a waste heat process into the district heating system through seasonal storage. No such system yet exists in Finland, but due to the widespread use of district heating and waste-to-energy processes, the potential for reducing carbon dioxide (CO2) emissions is great. Previous studies on the topic did not report the cost of heat produced through the seasonal waste heat storage system. In this study, we explicitly report the levelized cost of heat obtained from a BTES system. Both the emission reduction potential and its cost are novel and useful information for Finnish and Nordic district heating operators. Paste experimental studies analyze just a single storage system configuration, which may or may not be optimal. The parametric runs performed in the study show the performance range and tradeoffs available for optimal waste heat storage systems, which was not presented in the previous studies. Buildings 2020, 10, 205 3 of 19 Buildings 2020, 10, x 3 of 19 2. Materials and Methods 2. Materials and Methods This study on incineration waste heat recovery was performed using dynamic energy simulation. This study on incineration waste heat recovery was performed using dynamic energy A flowchart of the whole calculation process is presented in Figure1. simulation. A flowchart of the whole calculation process is presented in Figure 1.

FigureFigure 1.1. FlowchartFlowchart ofof thethe calculationcalculation process.process.

HeatingHeating demanddemand proﬁlesprofiles fromfrom aa previousprevious simulationsimulation studystudy werewere usedused asas inputinput datadata forfor energyenergy storagestorage calculations.calculations. VariousVarious system system conﬁgurations configurations were were tested tested and and out out of allof theall combinations,the combinations, a few a optimalfew optimal solutions solutions were were chosen chosen based based on on the the cost cost of of heat heat and and the the amount amount of of heat heatrecovered recovered fromfrom thethe storage.storage.

2.1.2.1. District Information TheThe goalgoal ofof thethe studystudy waswas to to analyze analyze the the utilization utilization of of waste waste heat heat from from an an incineration incineration plant plant in in a newa new planned planned residential residential district district in in the the Finnish Finnish city city of of Vaasa. Vaasa. Vaasa Vaasa is locatedis located in ain subarctic a subarctic climate climate of Finnishof Finnish climate climate zone zone II, which II, which is represented is represented by the by Finnish the Finnish Test Reference Test Reference Year 2012 Year (TRY2012) 2012 (TRY2012) weather dataweather of Vantaa data of [11 Vantaa]. The [11]. average The ambientaverage ambient air temperature air temperature in TRY2012 in TRY2012 is 5.6 ◦C. is The 5.6 °C. heating The heating degree daydegree value day (S17) value for (S17) Vaasa for in theVaasa reference in the periodreference (1980–2010) period (1980–2010) was 4469 Kd.was The 4469 district Kd. The comprised district apartmentcomprised buildings apartment built buildings according built to energyaccording performance to energy requirements performance of requirements the 2010 Finnish of buildingthe 2010 2 2 codeFinnish [12 ],building with a specificcode [12], annual with a heating specific demand annual ofheating 64.5 kWh demand/m . of With 64.5 130,000 kWh/m m2. Withof heated 130,000 floor m2 area,of heated this correspondsfloor area, this to 8.4corresponds GWh total to annual 8.4 GWh heating total demand. annual heating Space heating,demand. ventilation Space heating, and domesticventilation hot and water domestic (DHW) consumptionhot water (DHW) profiles consumption were obtained profiles from awere previous obtained study from [13], ina whichprevious an apartmentstudy [13], buildingin whichwas an apartment simulated building using the was IDA-ICE simulated 4.8 simulation using the IDA-ICE tool [14]. 4.8 The simulation different demands tool [14]. wereThe different combined demands into a single were hourly combined energy into demand a single profile, hourly which energy was demand normalized profile, to the which building was codenormalized requirement. to the building code requirement.

2.2.2.2. Energy SystemSystem TheThe plannedplanned districtdistrict waswas suppliedsupplied withwith wastewaste incineration heatheat andand conventional districtdistrict heatingheating (DH).(DH). AA TRNSYSTRNSYS modelmodel waswas createdcreated toto simulatesimulate thethe interactioninteraction ofof aa seasonal borehole thermalthermal energy storagestorage (BTES)(BTES) systemsystem withwith thethe conventionalconventional districtdistrict heatingheating networknetwork and the buildings themselves. TheThe BTESBTES systemsystem waswas aa fieldfield ofof boreholesboreholes drilleddrilled intointo thethe groundground andand fittedfitted withwith U-tubeU-tube heatheat transfertransfer pipes.pipes. Heat exchangers (HX) (HX) between between the the DH grid and the BTES were ideal (Type 91) withwith 95%95% efficiency. The BTES system was modelled in TRNSYS using Type 557a. The heat capacity of the ground was 2240 kJ/m3K with a high heat conductivity of 3.5 W/mK [15]. The top surface of the BTES Buildings 2020, 10, 205 4 of 19 efficiency. The BTES system was modelled in TRNSYS using Type 557a. The heat capacity of the Buildingsground 2020 was, 10 2240, x kJ/m3K with a high heat conductivity of 3.5 W/mK [15]. The top surface of the4 BTES of 19 system was covered by a 10 cm polystyrene (Styrofoam) insulation layer to reduce heat losses. Thermal system was covered by a 10 cm polystyrene (Styrofoam) insulation layer to reduce heat losses. conductivity of the Styrofoam was 0.03 W/mK [16]. The sides of the BTES system and the boreholes Thermal conductivity of the Styrofoam was 0.03 W/mK [16]. The sides of the BTES system and the themselves were not insulated. The starting temperature of the ground was 5.6 C[15]. The borehole boreholes themselves were not insulated. The starting temperature of the ground◦ was 5.6 °C [15]. The width was 15 cm and the inner diameter of the U-tube pipe was 3.2 cm. borehole width was 15 cm and the inner diameter of the U-tube pipe was 3.2 cm. The boreholes in the BTES system were distributed uniformly in a cylindrical volume. Some of The boreholes in the BTES system were distributed uniformly in a cylindrical volume. Some of the boreholes were connected in series, such that the hot water for charging was fed into the center the boreholes were connected in series, such that the hot water for charging was fed into the center of the storage and taken out at the outside edge, going through several boreholes in the process. of the storage and taken out at the outside edge, going through several boreholes in the process. During discharging, the flow direction was reversed, such that cold water was supplied from the edges. During discharging, the flow direction was reversed, such that cold water was supplied from the edges. A higher number of series-connected boreholes increases heat transfer efficiency, while reducing the A higher number of series-connected boreholes increases heat transfer efficiency, while reducing the total flow rate and power. It also creates a larger radial temperature difference in the storage, reducing total flow rate and power. It also creates a larger radial temperature difference in the storage, reducing heat losses. This is visualized in Figure2, which shows a borehole field and the principle of series heat losses. This is visualized in Figure 2, which shows a borehole field and the principle of series connection between boreholes (in charging mode). connection between boreholes (in charging mode).

Figure 2. The operational principle of borehole therma thermall energy storage (BTES). The horizontal BTES cross-section (left) shows the borehole grid and the radial heat distribution with a hot core and cooler edges. The The series series connection connection of of boreholes boreholes ( (rightright)) results results in in a a larger temperature change in the heat transfer fluid. ﬂuid.

The inlet flow flow sheds a part of its energy into the central borehole, cooling cooling down down in in the the process. process. The flow flow is then redirected to another borehole, cool coolinging down more and finally finally exiting from the outer borehole at the lowest temperature. The efficiency efficiency of the BTES was defineddefined as, Equation (1):(1):

η = Eextracted/Echarged (1) η = Eextracted/Echarged (1) where Eextracted is the amount of energy taken out of the storage over a year and Echarged is the energy wherecharged Eextracted into theis storage the amount over the of energy same time. taken out of the storage over a year and Echarged is the energy chargedDuring into the the heating storage season, over the all same of the time. heat in the waste incineration plant was used elsewhere in the city,During but thein summer heating season,there was all ofexcess the heat heat in availa the wasteble to incineration be used in plantthe new was district. used elsewhere According in the to thecity, local but in utility, summer there there was was 45 excess GWh heat of availableexcess heat to be available used in the from new the district. incineration According plant tothe during local summer.utility, there This was was 45 much GWh ofmore excess than heat even available the annual from theheating incineration demand plant of the during new summer.district. Thus, This was all heatingmuch more demand than evenduring the summer annual heatingwas covered demand by ofthe the waste new heat district. and Thus, any excess all heating could demand be charged during in thesummer seasonal was energy covered storage by the wastesystem. heat The and monthl any excessy heating could demands be charged are inshown the seasonal in Figure energy 3. storage system. The monthly heating demands are shown in Figure3. Buildings 2020, 10, 205 5 of 19 BuildingsBuildings 2020 2020, ,10 10, ,x x 55 ofof 1919

1,2001,200 CoventionalCoventional DHDH ConventionalConventional DHDH 1,0001,000 800800 600600 WasteWaste heatheat DH DH 400400 200200 Heating demand (MWh) demand Heating Heating demand (MWh) demand Heating 00 123456789101112123456789101112 MonthMonth

FigureFigure 3.3.3. Monthly MonthlyMonthly heating heatingheating demand demanddemand in inin the thethe residential residentialresidential district didistrictstrict without withoutwithout seasonal seasonalseasonal energy energyenergy storage. storage.storage. The blue TheThe colorblueblue color showscolor showsshows when whenwhen heating heatingheating is provided isis providedprovided by conventional byby coconventionalnventional district districtdistrict heating heatingheating and the andand orange thethe orangeorange shows showsshows when wastewhenwhen wasteheatwaste is heatheat directly isis directlydirectly utilized. utilized.utilized.

DuringDuring thethe winter winter months, months, thethe monthly monthly heatingheating demanddemdemandand inin thethe district district is is about about 1000 1000 MWh, MWh, while while duringduring summersummer thethe monthlymonthly demanddemand remainsremains belowbelowbelow 500500500 MWh.MWh.MWh. WasteWaste heatheat waswas availableavailable fromfrom thethe startstart ofof MayMay toto thethe endend ofof August,August, whichwhich limitedlimited thethe chargingcharging timetime ofofof the thethe BTES BTESBTES to toto four fourfour months. months.months. For ForFor this thisthis reason, reason,reason, the thethe charging chargingcharging ﬂowrate flowrateflowrate to thetoto thethe BTES BTESBTES was waswas 1600 16001600 kg/ h kg/hkg/h per boreholeperper boreholeborehole loop, loop,loop, while whilewhile the discharge thethe didischargescharge ﬂowrate flowrateflowrate was was lowerwas lowerlower at 1200 atat 12001200 kg/h kg/hkg/h per borehole perper boreholeborehole loop. loop.loop. TheThe newnewnew district districtdistrict utilized utilizedutilized a a lowa lowlow temperature temperaturetemperature local locallocal heating heatingheating grid gridgrid with withwith 70 ◦ 70C70 supply°C°C supplysupply temperature temperaturetemperature and 30andand◦ C 3030 return °C°C returnreturn temperature temperaturetemperature from thefromfrom buildings. thethe buildings.buildings. The BTES TheThe wasBTESBTES connected waswas connectedconnected to the conventional toto thethe conventionalconventional district heatingdistrictdistrict heating systemheating insystemsystem series, inin as series,series, shown asas in shownshown Figures inin 4 FiguresFigures and5. 44 andand 5.5.

FigureFigure 4.4. BTESBTES chargingcharging modemode (summer).(summer). Figure 4. BTES charging mode (summer). Buildings 2020, 10, 205 6 of 19 Buildings 2020, 10, x 6 of 19

FigureFigure 5.5. BTESBTES dischargingdischarging modemode (winter).(winter).

InIn summer summer (Figure (Figure4), 4), waste waste heat heat was was supplied supplied directly directly to theto the low low temperature temperature heating heating grid grid and and to chargeto charge the the BTES. BTES. In winter In winter (Figure (Figure5), the 5), return the return ﬂow fromflow thefrom local thegrid local was grid preheated was preheated using theusing BTES the andBTES then and heated then heated to the requiredto the required temperature temperature level (70 leve◦C)l (70 with °C) the with conventional the conventional district district heating. heating. Heat transferHeat transfer and losses and losses in the in district the district heating heating pipes pipes were were not modelled, not modelled, only only the connection the connection between between the BTESthe BTES and otherand other grids grids was modelled. was modelled. The carbon The carbon dioxide dioxide emission emission factorof factor the conventional of the conventional district heatingdistrict heating was assumed was assumed to match to thematch national the national average average of 194 of kg-CO 194 kg-CO2/MWh2/MWh [17]. [17]. Heat Heat produced produced by otherwiseby otherwise wasted wasted incineration incineration heat heat was was assumed assumed to have to have no emissions. no emissions.

2.3.2.3. CostCost CalculationCalculation TheThe costcost analysisanalysis waswas limitedlimited toto thethe directdirect investmentinvestment costscosts ofof thethe BTESBTES system,system, whichwhich werewere thethe costcost ofof boreholes and piping piping at at 33.5 33.5 €/m€/m and and the the cost cost of ofthe the BTES BTES polystyrene polystyrene insulation insulation cover cover at 83.3 at 3 83.3€/m3€./ mThe. Thecover cover was was 10 10cm cm thick thick in inevery every scenario. scenario. The The district district heating heating grid grid was was assumed toto bebe installedinstalled regardlessregardless ofof the the BTES BTES system system and and no no extra extra costs costs were were attached attached to to it. it. TheThe BTESBTES systemsystem waswas assumedassumed toto requirerequire annualannual maintenancemaintenance valuedvalued atat 0.5%0.5% ofof thethe investmentinvestment cost.cost. TheThe levelizedlevelized costcost ofof heatheat (LCOH)(LCOH) was was calculated calculated over over a 25-yeara 25-year period. period. Discounting Discounting was was done done using using a 3% a interest3% interest rate andrate 2%and energy 2% energy price price escalation escalation [18]. [18].

2.4. Parametric Runs 2.4. Parametric Runs For the new storage-based heating system to be economically feasible, it needs to be competitive For the new storage-based heating system to be economically feasible, it needs to be competitive with the conventional heat generation system. According to district heating operators, a levelized cost of with the conventional heat generation system. According to district heating operators, a levelized heat below 20 €/MWh would be preferable for widespread adoption [19]. For comparison, the after-tax cost of heat below 20 €/MWh would be preferable for widespread adoption [19]. For comparison, the consumer price for district heating in Helsinki in 2020 was 35–66 €/MWh [20]. Parametric runs were after-tax consumer price for district heating in Helsinki in 2020 was 35–66 €/MWh [20]. Parametric performed with various BTES conﬁgurations, to see how the economics changed. The variables were runs were performed with various BTES configurations, to see how the economics changed. The 3 the volume of the rock used for storage (VBTES, 300,000 and 600,000 m ), the height-to-width ratio of variables were the volume of the rock used for storage (VBTES, 300,000 and 600,000 m3), the height-to- the cylindrical storage (hratio, 0.5, 1.0 and 2.0 m/m), the areal density of boreholes (BH) on the BTES width ratio of the cylindrical storage (hratio, 0.5, 1.0 and 2.0 m/m), the areal density of boreholes (BH) surface, i.e., how many boreholes per area (BH density, 0.05 and 0.10 1/m2) and the number of boreholes on the BTES surface, i.e., how many boreholes per area (BH density, 0.05 and 0.10 1/m2) and the connected in series (Nseries, 3, 6, 9). The settings of the parametric runs are displayed in Table1. number of boreholes connected in series (Nseries, 3, 6, 9). The settings of the parametric runs are displayed in Table 1.

Buildings 2020, 10, 205 7 of 19

Table 1. Parameters of the diﬀerent borehole thermal energy storage (BTES) conﬁgurations tested.

V h BH Density Case BTES ratio N (m3) (m/m) (1/m2) series 1 300,000 0.5 0.05 3 2 300,000 0.5 0.1 3 3 300,000 0.5 0.05 6 4 300,000 0.5 0.1 6 5 300,000 0.5 0.05 9 6 300,000 0.5 0.1 9 7 300,000 1 0.05 3 8 300,000 1 0.1 3 9 300,000 1 0.05 6 10 300,000 1 0.1 6 11 300,000 1 0.05 9 12 300,000 1 0.1 9 13 300,000 2 0.05 3 14 300,000 2 0.1 3 15 300,000 2 0.05 6 16 300,000 2 0.1 6 17 300,000 2 0.05 9 18 300,000 2 0.1 9 19 600,000 0.5 0.05 3 20 600,000 0.5 0.1 3 21 600,000 0.5 0.05 6 22 600,000 0.5 0.1 6 23 600,000 0.5 0.05 9 24 600,000 0.5 0.1 9 25 600,000 1 0.05 3 26 600,000 1 0.1 3 27 600,000 1 0.05 6 28 600,000 1 0.1 6 29 600,000 1 0.05 9 30 600,000 1 0.1 9 31 600,000 2 0.05 3 32 600,000 2 0.1 3 33 600,000 2 0.05 6 34 600,000 2 0.1 6 35 600,000 2 0.05 9 36 600,000 2 0.1 9

The parametric runs included all combinations of the mentioned variables. Under these shapes and sizes, the diameter of the BTES varied between 58 and 115 m, while the depth of the BTES varied between 46 and 145 m. The number of boreholes was between 130 and 656 for the smaller BTES size and between 207 and 1042 for the larger BTES size. The volume of 300,000 m3 (small) and 600,000 m3 (large) corresponded to heat storage capacities of 3.5 GWh and 7 GWh, respectively. Before accounting for losses, these small and large BTES systems should be able to store enough heat to cover 50% and 100% of the winter heating demand, respectively. The height-to-width ratio has an effect on heat losses through the uninsulated sides and the insulated top, while the BH density affects the heat transfer power. Having more boreholes connected in series increases the potential temperature change achieved by the underground heat collectors, but reduces the total flow and heat transfer power in the system. Buildings 2020, 10, 205 8 of 19

3. Results

3.1. All Cases The results of the parametric run are presented in Table2, arranged according to levelized cost of heat.

Table 2. Results of the parametric run arranged by levelized cost of heat (LCOH). Highlighted in bold are three cases determined through the Pareto optimality condition based on maximal heat output and minimal LCOH. See Section 2.4.

BH BTES Heat BTES Waste V h N LCOH Case BTES ratio Density series Output Eﬃciency Fraction (m3) (m/m) (1/m2) - (MWh) % % (€/MWh) 1 300,000 0.5 0.05 3 2761 60.1 53.9 10.5 7 300,000 1 0.05 3 2652 61.0 52.6 10.5 3 300,000 0.5 0.05 6 2607 60.2 52.0 11.1 13 300,000 2 0.05 3 2368 57.8 49.2 11.6 9 300,000 1 0.05 6 2366 60.3 49.2 11.9 5 300,000 0.5 0.05 9 2385 59.1 49.4 12.2 4 300,000 0.5 0.1 6 4220 67.8 71.3 12.9 8 300,000 1 0.1 3 4119 67.7 70.1 13.0 2 300,000 0.5 0.1 3 4181 66.3 70.8 13.0 10 300,000 1 0.1 6 4046 69.2 69.2 13.2 6 300,000 0.5 0.1 9 4076 67.9 69.5 13.4 19 600,000 0.5 0.05 3 4186 57.1 70.9 13.6 14 300,000 2 0.1 3 3887 65.9 67.3 13.6 25 600,000 1 0.05 3 4060 58.1 69.4 13.6 21 600,000 0.5 0.05 6 4078 57.9 69.6 14.0 11 300,000 1 0.05 9 2007 57.5 44.9 14.1 12 300,000 1 0.1 9 3783 69.0 66.0 14.2 16 300,000 2 0.1 6 3600 66.8 63.9 14.7 27 600,000 1 0.05 6 3774 58.3 65.9 14.8 15 300,000 2 0.05 6 1857 54.2 43.1 14.8 31 600,000 2 0.05 3 3660 55.0 64.6 14.9 23 600,000 0.5 0.05 9 3804 57.2 66.3 15.1 29 600,000 1 0.05 9 3306 56.5 60.4 16.9 18 300,000 2 0.1 9 3135 65.5 58.3 17.0 33 600,000 2 0.05 6 3082 53.8 57.7 17.8 22 600,000 0.5 0.1 6 5717 62.8 89.1 18.9 28 600,000 1 0.1 6 5595 64.6 87.7 19.0 24 600,000 0.5 0.1 9 5644 63.3 88.2 19.2 26 600,000 1 0.1 3 5501 62.4 86.5 19.3 20 600,000 0.5 0.1 3 5533 60.7 86.9 19.5 30 600,000 1 0.1 9 5375 64.6 85.0 19.9 32 600,000 2 0.1 3 5277 60.8 83.9 20.0 17 300,000 2 0.05 9 1380 48.1 37.4 20.1 34 600,000 2 0.1 6 5157 62.6 82.4 20.5 36 600,000 2 0.1 9 4684 62.0 76.8 22.8 35 600,000 2 0.05 9 2352 48.7 49.0 23.5

The reported results are the heat extracted from the BTES during the fifth year (BTES heat output), the fraction of stored heat that was utilized during the fifth year (BTES efficiency), the fraction of annual heating demand covered by incineration waste heat during the fifth year and the levelized cost of heat over the whole 25-year calculation period (LCOH). There was a correlation between higher cost and higher heat output from the storage, but there were also exceptions to the rule. The LCOH in the examined cases ranged from 10.5 to 23.5 €/MWh. Compared with the 56 €/MWh district heating price in Vaasa, the cost of heat was low enough to be of economic interest to utilities. The heat output from BTES was 1380–5717 MWh. The direct utilization Buildings 2020, 10, x 9 of 19 BuildingsBuildings2020 ,202010, 205, 10, x 9 9of of 19 19 of economic interest to utilities. The heat output from BTES was 1380–5717 MWh. The direct utilizationof economic of waste interest heat to during utilities. summer The heatwas 1758output MWh, from so BTES the BTES was increased1380–5717 waste MWh. heat The use direct by of waste heat during summer was 1758 MWh, so the BTES increased waste heat use by 78–325%. 78–325%.utilization The of annual waste efficiencyheat during of thesummer BTES variedwas 1758 between MWh, 48.1% so the and BTES 69.2% increased and the waste total wasteheat use heat by The annual efficiency of the BTES varied between 48.1% and 69.2% and the total waste heat utilization utilization78–325%. factor The annual (over theefficiency whole year)of the wasBTES 37.4–89. varied1%. between The charging 48.1% and temperature 69.2% and inthe all total cases waste was heat75 factor (over the whole year) was 37.4–89.1%. The charging temperature in all cases was 75 C. °C.utilization factor (over the whole year) was 37.4–89.1%. The charging temperature in all cases◦ was 75 Three solutions (cases 1, 4 and 22) were presented in a bold font to show the most cost-effective °C.Three solutions (cases 1, 4 and 22) were presented in a bold font to show the most cost-effective casescases for forThree di differentfferent solutions energy energy (cases output output 1, 4 levels. andlevels. 22) TheseThese were solutionssolutipresentedons are arein aalsoalso bold shown shown font to in inshow Figure Figure the 6,6 mostalongside, alongside cost-effective all all the the otherothercases examined examined for different cases. cases. energy output levels. These solutions are also shown in Figure 6, alongside all the other examined cases.

FigureFigure 6. 6.Three Three highlighted highlighted casescases basedbased on Pareto op optimaltimal selection selection from from all all simulated simulated cases. cases. Figure 6. Three highlighted cases based on Pareto optimal selection from all simulated cases. TheThe solutions solutions were were arrangedarranged accordingaccording to LCOH and and BTES BTES heat heat utilization utilization and and the the three three The solutions were arranged according to LCOH and BTES heat utilization and the three highlightedhighlighted solutions solutions were were selected selected based based on theon Paretothe Pareto optimality optimality concept. concept. A Pareto A Pareto optimal optimal solution highlighted solutions were selected based on the Pareto optimality concept. A Pareto optimal is onesolution where is one one where property one property of a solution of a solution cannot becannot improved be improved without without making making another another worse. worse. Thus, solution is one where one property of a solution cannot be improved without making another worse. threeThus, points three were points chosen were chosen by trying by trying to minimize to mini LCOHmize LCOH and maximizeand maximize BTES BTES heat heat at the at the same same time. Thus, three points were chosen by trying to minimize LCOH and maximize BTES heat at the same Detailstime. ofDetails the highlighted of the highlighted solutions solutions are reported are reported in Figure in Figure6. 6. time.The Details figure of shows the highlighted three clusters solutions of solutions, are reported which in Figure are analyzed 6. further in the following The ﬁgure shows three clusters of solutions, which are analyzed further in the following paragraphs. paragraphs.The figure The levelized shows three cost ofclusters heat was of determsolutions,ined which for all arecases. analyzed Figure 7further shows inthe the LCOH following as a The levelized cost of heat was determined for all cases. Figure7 shows the LCOH as a function functionparagraphs. of heat The discharged levelized fromcost of the heat BTES was during determ theined fifth for year all cases. of operation, Figure 7 separated shows the by LCOH storage as a of heat discharged from the BTES during the ﬁfth year of operation, separated by storage size and sizefunction and borehole of heat density.discharged from the BTES during the fifth year of operation, separated by storage boreholesize and density. borehole density.

Figure 7. Levelized cost of heat as a function of the fifth-year heat output from the BTES for all FigureexaminedFigure 7. Levelized 7. scenarios. Levelized cost The ofcost heatresults of asheat are a function asclassified a function of theaccording ﬁfth-yearof the tofifth-year BTES heat outputvolume heat from outputand theborehole from BTES the density. for BTES all examined for all scenarios.examined The scenarios. results are The classiﬁed results are according classified to a BTESccording volume to BTES and volume borehole and density. borehole density. Buildings 2020, 10, 205 10 of 19 Buildings 2020, 10, x 10 of 19

ThreeThree di ffdifferenterent clusters clusters could could be be identified identified along along these these criteria.criteria. The clusters clusters were were separated separated by byBTES BTES volume volume and and borehole density. TheThe middlemiddle clustercluster actuallyactually combined combined two two di differentfferent sets sets of of solutions:solutions: (i) (i) those those with with small small volume volume and and high high borehole borehole density density and and (ii) (ii) those those with with large large volume volume and and lowlow borehole borehole density. density. Doubling Doubling the the volume volume had had a similar a similar eff effectect on on cost cost and and heat heat recovery recovery as as doubling doubling thethe borehole borehole density. density. Both Both measures measures increased increased the number the number of boreholes, of boreholes, which increased which increased heat transfer heat capacitytransfer and capacity energy and provision, energy butprovisio also raisedn, but thealso cost raised of heating. the cost Increasingof heating. the Increasing number ofthe boreholes number of hadboreholes the same had effect the under same both effect BTES under sizes. both Each BTES cluster sizes. had Each a negative cluster had linear a nega correlationtive linear between correlation heat costbetween and energy heat output.cost and The energy difference output. in The investment difference costs in withininvestment each clustercosts within was minimal, each cluster which was wasminimal, why increased which was energy why utilization increased directlyenergy utilization reduced the directly cost of reduced heating. the cost of heating. TheThe same same results results are presentedare presented in Figure in Figure8, but separated8, but separated by height-to-width by height-to-width ratio and theratio number and the ofnumber boreholes of inboreholes series. in series.

FigureFigure 8. 8.Levelized Levelized cost cost of of heat heat as as a functiona function of of the the ﬁfth-year fifth-year heat heat output output from from the the BTES BTES for for all all examinedexamined scenarios. scenarios. The The results results are classiﬁed are classified according according to BTES to height-to-width BTES height-to-width ratio and theratio number and the of boreholesnumber of connected boreholes inconnected series. in series.

InIn each each line line of of solutions, solutions, the the most most expensive expensive ones ones were were those those with with a h aratio hratioof of 2, 2, meaning meaning BTES BTES systemssystems that that were were deep deep and and narrow. narrow. This This could could also also be be seen seen in in Table Table2, where2, where the the five five most most expensive expensive solutionssolutions had had a narrowa narrow BTES. BTES. If 9If boreholes9 boreholes were were connected connected in in series series (green), (green), the the best best shape shape was was a a widewide one one (h ratio(hratio, 0.5)., 0.5). Conversely, Conversely, for for a a narrow narrow BTES BTES (h (hratioratio, 2), the most cost-effective cost-effective method was was to toconnect connect only 3 boreholesboreholes inin series.series. WhenWhen manymany boreholes boreholes were were connected connected in in series, series, the the total total heat heat transfertransfer in in and and out out of of the the storage storage went went down, down, because because the the total total flowflow raterate waswas lowered.lowered. This This might might be becompensated compensated by by increasing increasing the total numbernumber ofof boreholes,boreholes, whichwhich could could be be done done by by reducing reducing the the height-to-widthheight-to-width ratio ratio or or by by increasing increasing the the borehole borehole density. density. In In each each solution solution cluster, cluster, the the wide wide BTES BTES shapeshape was was the the most most cost-e cost-effective,ffective, but but the the diff differenceerence to theto the even even shape shape (hratio (hratio, 1), 1) was was not not great. great. There There seemedseemed to to be be a sweeta sweet spot spot of of borehole borehole connectivity. connectivity. Having Having more more boreholes boreholes (higher (higher borehole borehole density) density) favoredfavored connecting connecting 6 boreholes 6 boreholes in in series, series, instead instead of of 3. 3. More More seriality seriality resulted resulted in in a highera higher temperature temperature changechange of theof the heat heat transfer transfer fluid, fluid, potentially potentially reducing reduci theng the need need for primingfor priming by district by district heat. heat. However, However, if theif totalthe total flow rate,flow i.e.,rate, the i.e., number the number of boreholes of boreholes was too was low, too the low, need the for conventionalneed for conventional district heating district didheating not change. did not A balancechange. wasA balance required was between required the be outputtween temperature the output temperature and flow rate and on theflow supply rate on sidethe and supply power side demand and power and temperaturedemand and requirement temperature on requirement the demand on side. the demand However, side. the importanceHowever, the ofimportance the temperature of the provided temperature by the provided BTES would by the rise BTES ifthe would storage rise wasif the connected storage was to theconnected DH grid to in the parallelDH grid instead in parallel of in series instead (as of in in this series study). (as in this study). FigureFigure9 shows 9 shows the the annual annual BTES BTES e efficiencyfficiency (fifth(fifth year) in in all all cases cases as as a afunction function of ofuseful useful heat heatextraction. extraction. BuildingsBuildings2020 2020, 10, ,10 205, x 1111 of of 19 19 Buildings 2020, 10, x 11 of 19

Figure 9. Seasonal storage efficiency as a function of useful heat output from the BTES. FigureFigure 9. 9.Seasonal Seasonal storagestorage eefficiencyﬃciency as as a a function function of of useful useful heat heat output output from from the the BTES. BTES.

TheTheThe attained attainedattained e ffi efficiencyefficiencyciency range rangerange in all inin configurations allall configurconfigurationsations was 48–69%, waswas 48–69%,48–69%, with a with medianwith aa median ofmedian 61%. of Theof 61%.61%. highest TheThe ehighestffihighestciency efficiency efficiency was obtained was was obtained obtained in the small in in the the storage. small small storag storag Thee. storagee. The The storage storage efficiency efficiency efficiency was generallywas was generally generally greater greater greater in the in in smallthethe small small storage storage storage than than than in the in in the large the large large storage, storage, storage, but but but with with with a lower a a lower lower energy energy energy output. output. output. With With With all all all other other other things things things being being being equal,equal,equal, aa a largelarge large storagestorage storage systemsystem system shouldshould should havehave have aa a higherhigher higher eefficiency ffiefficiencyciency becausebecause because ofof ofa a amore more more favorable favorable favorablesurface surface surface area-to-volumearea-to-volumearea-to-volume ratio. ratio.ratio. However,However, However, thisthis this isis is also alsoalso influenced ininfluencedfluenced by byby the thethe heating heatingheating demand. demand.demand. At AtAt the thethe end endend of ofof the thethe heatingheatingheating season, season, season, the the the base base base temperaturetemperature temperature level level level remained remained remained higher higher higher in in in thethe the largelarge large storagestorage storage thanthan than inin in thethe the smallsmall small one.one. one. ThisThisThis increased increased increased the the the heat heat heat losses losses losses in thein in the largerthe larger larger storage stor storage system.age system. system. The smallThe The small small storage storage storage system system system was drained was was drained drained more completely,moremore completely, completely, resulting resulting resulting in increased in in increased increased efficiency. efficiency. efficiency. It was moreIt It was was cost-e more moreffective cost-effective cost-effective to size the to to storagesize size the the tostorage storage cover ato to smallercovercover a parta smaller smaller of the part part heating of of the the load. heating heating On theload. load. other On On hand,the the other other a larger hand, hand, storage a a larger larger was storage storage required was was to required coverrequired a larger to to cover cover part a a oflargerlarger heating part part demand of of heating heating by wastedemand demand heat. by by waste waste heat. heat. TheTheThe BTES BTES BTES configuration configuration configuration influenced influenced influenced the the the temperature temperature temperature than than than can can can be beobtained be obtained obtained in thein in the storage.the storage. storage. Figure Figure Figure 10 shows1010 shows shows the storagethe the storage storage efficiency efficiency efficiency vs. thevs. vs. the annual the an annualnual maximum maximum maximum temperature temperature temperature of the of of BTES.the the BTES. BTES.

Figure 10. BTES efficiency as a function of maximum storage temperature. The three best cases are FigureFigure 10. 10.BTES BTES eefficiencyﬃciency asas a a function function of of maximum maximum storage storage temperature. temperature. TheThe threethree bestbest casescases areare highlighted by circles and case numbers. highlightedhighlighted by by circles circles and and case case numbers. numbers.

TheTheThe resultsresults results showed showed a aa clear clear correlation correlation correlation between between between im im improvedprovedproved efficiency efficiency efficiency and and and rising rising rising temperature. temperature. temperature. One One Onecouldcould could expectexpect expect thethe the oppositeopposite opposite results,results, results, wherewhere where aa a highhigh high temperaturetemperature increases increasesincreases heat heatheat losseslosses and andand lowers lowerslowers eefficiency.ffiefficiency.ciency. However, However, However, the the the temperature temperature temperature of of of the the the storagestorage storage setset set aa a maximummaximum maximum limitlimit limit onon on thethe the extractionextraction extraction eefficiency. efficiency.fficiency. OnceOnceOnce the the the demand-side demand-side demand-side flowflow flow waswas was heatedheated heated upup up toto to the the the BTES BT BTESES temperature,temperature, temperature, increasingincreasing increasing thethe the flowflow flow raterate rate onon on the the the storagestoragestorage sideside side did did did not not not increase increase increase heat heat heat extraction. extraction. extraction. ThisThis Thiswas was wasdemonstrated demonstrated demonstrated inin inFigure Figure Figure 10 10, 10,, where where where the the the lowest lowest lowest temperaturetemperaturetemperature resulted resulted resulted in in in the the the lowest lowest lowest e efficiency.ffi efficiency.ciency. In In In all all all the the the cases, cases, cases, the the the size size size of of of the the the seasonal seasonal seasonal energy energy energy storage storage storage waswas so so large large that that the the conductive conductive heat heat losses losses caus causeded by by high high temperatures temperatures were were dominated dominated by by the the increasedincreased ability ability to to meet meet the the heating heating demands. demands. Buildings 2020, 10, 205 12 of 19

wasBuildings so large 2020, 10 that, x the conductive heat losses caused by high temperatures were dominated by12 theof 19 Buildings 2020, 10, x 12 of 19 increased ability to meet the heating demands. 3.2. Development of BTES State 3.2.3.2. DevelopmentDevelopment ofof BTES BTES State State Figure 11 shows how the heat storage efficiency of the BTES system developed during the first Figure 11 shows how the heat storage efficiency of the BTES system developed during the first fiveFigure years of 11 simulated shows how operation. the heat storage efficiency of the BTES system developed during the first fivefive years years of of simulated simulated operation. operation.

Figure 11. Development of BTES storage efficiency in all the scenarios during the first five years of FigureFigure 11. 11. DevelopmentDevelopment of BTES storage effi efficiencyciency in in all all the the scenarios scenarios during during the the first first five five years years of operation. ofoperation. operation. During the first year, most of the excess energy went to raising the BTES temperature from the DuringDuring the the first first year, year, most most of of the the excess excess energy energy went went to to raising raising the the BTES BTES temperature temperature from from the the ambient 5.6 °C to the minimum useful temperature of 30 °C, since no heat pumps were included in ambientambient 5.6 5.6◦ C°C to to the the minimum minimum useful useful temperature temperature of 30of ◦30C, °C, since since no heatno heat pumps pumps were were included included in the in the system. This resulted in a storage efficiency of less than 20% in every case. The largest rise in system.the system. This resultedThis resulted in a storage in a storage efficiency efficiency of less thanof less 20% than in every 20% case.in every The case. largest The rise largest in effi ciencyrise in efficiency was observed between the first two years, because it was already possible to use the stored wasefficiency observed was between observed the between first two the years, first because two years, it was because already it was possible already to use possible the stored to use heat the during stored heat during the second year. The improvement was smaller every consecutive year as the system theheat second during year. the Thesecond improvement year. The wasimprovement smaller every was consecutive smaller every year consecutive as the system year started as the to system reach started to reach equilibrium. In some cases, the steady-state condition was reached faster than in equilibrium.started to reach In some equilibrium. cases, the In steady-state some cases, condition the steady-state was reached condition faster than was in reached others. faster For example, than in others. For example, in the highest efficiency case (case 10), the efficiency improved from 63% to 69% inothers. the highest For example, efficiency in casethe highest (case 10), efficiency the efficiency case (case improved 10), the from efficiency 63% to 69%improved between from the 63% third to and69% between the third and fifth year. In the lowest efficiency case (case 35), the improvement was from fifthbetween year. Inthe the third lowest and e fifthfficiency year. case In the (case lowest 35), the effi improvementciency case (case was from35), the 31% improvement to 49%. The latter was casefrom 31% to 49%. The latter case involved a large storage system with low borehole density and maximal involved31% to 49%. a large The storage latter case system involved with low a large borehole storage density system and with maximal low borehole seriality, density which and reduced maximal the seriality, which reduced the charging power. With a proper design, i.e., a high enough heat transfer chargingseriality, power.which reduced With a proper the charging design, power. i.e., a high With enough a proper heat design, transfer i.e., power a high relative enough to heat the storagetransfer power relative to the storage volume, the BTES system in the highly conductive rocky Finnish ground volume,power relative the BTES to systemthe storage in the volume, highly the conductive BTES system rocky in Finnish the highly ground conductive could be rocky running Finnish close ground to the could be running close to the full efficiency after three years of operation. fullcould effi ciencybe running after close three to years the full of operation. efficiency after three years of operation. Figure 12 shows the development of the waste heat utilization factor over years of operation. FigureFigure 12 12 shows shows the the development development of of the the waste waste heat heat utilization utilization factor factor over over years years of of operation. operation.

FigureFigure 12. 12.Development Development ofof thethe amount amount ofof heat heat demand demand covered covered by by waste waste heat heat during during the the ﬁrst first ﬁve five Figure 12. Development of the amount of heat demand covered by waste heat during the first five yearsyears of of operation. operation. years of operation. The cases with a larger storage system tended to have a higher waste heat utilization than those The cases with a larger storage system tended to have a higher waste heat utilization than those with a smaller storage system. This was opposite to the BTES efficiency, which was increased for with a smaller storage system. This was opposite to the BTES efficiency, which was increased for smaller storages. In other words, a small storage system allowed a more effective use of the stored smaller storages. In other words, a small storage system allowed a more effective use of the stored waste heat, but reduced the potential to replace conventional district heating. Conversely, in a large waste heat, but reduced the potential to replace conventional district heating. Conversely, in a large Buildings 2020, 10, 205 13 of 19

The cases with a larger storage system tended to have a higher waste heat utilization than those with a smaller storage system. This was opposite to the BTES efficiency, which was increased for smaller storages. In other words, a small storage system allowed a more effective use of the stored Buildings 2020, 10, x 13 of 19 waste heat, but reduced the potential to replace conventional district heating. Conversely, in a large BTES, the high temperature levels could be retainedretained for longer, increasingincreasing thethe demand met by waste heat. The minimum first first year levels indicated that without seasonal thermal energy storage, less than 20% of the annual heatingheating demanddemand couldcould bebe metmet byby wastewaste heat.heat. Figure 13 shows the operational temperature range in the BTES during the last year for for all all cases. cases.

Figure 13.13. The operational temperature range of the BTES in each studied case during during the the fifth ﬁfth year. year. TheThe bestbest casescases areare identiﬁedidentified byby greengreen color.color.

ItIt alsoalso showedshowed howhow muchmuch energyenergy waswas dischargeddischarged fromfrom thethe storagestorage overover thethe year.year. BothBoth thethe maximum temperature and the dischargeddischarged heatheat ﬂuctuatedfluctuated upup andand downdown systematically.systematically. This waswas caused by the number of boreholes, which is related to the heat transfer capacity of the BTES system. InIn odd-numbered cases, cases, the the borehole borehole density density was was 0.05 0.05 1/m 1/m2, 2while, while in ineven-numbered even-numbered cases cases it was it was 0.1 0.11/m 12/. mAs2. it As could it could be assumed, be assumed, with with more more boreholes, boreholes, more more energy energy was wascharged charged into intothe ground the ground and andthe temperature the temperature levels levels were were higher. higher. The The annual annual temperature temperature change change in in the the BTES BTES was was also also directly related toto the the number number of of boreholes, boreholes, since since more more boreholes boreholes resulted resulted in more in heatingmore heating power. Thispower. allowed This chargingallowed charging the BTES the to higherBTES to temperature higher temperature levels, while levels, also while increasing also increasing the discharging the discharging rate and thusrate loweringand thus lowering the temperature the temperature at the end at the of the end heating of the heating season. season. Cases 31,Cases 33 31, and 33 35 and demonstrated 35 demonstrated how increasinghow increasing borehole borehole seriality seriality from from 3 to 6 3 to to 9 6 in to a 9 narrow in a narrow BTES reducedBTES reduced both the both temperature the temperature levels andlevels recovered and recovered energy. energy. Cases Cases 32, 34 32, and 34 36and displayed 36 displayed the samethe same trend, trend, but but with with a higher a higher number number of boreholes.of boreholes. This This happened happened with with smaller smaller BTES BTES sizes sizes as well,as well, in casesin cases 13 13 to 18.to 18.

3.3. Performance of the Best Cases Figure 14 showsshows thethe monthlymonthly energyenergy ﬂowsflows andand thethe monthlymonthly averageaverage BTESBTES temperaturetemperature for thethe threethree ParetoPareto optimaloptimal systemsystem conﬁgurationsconfigurations (cases(cases 1,1, 44 andand 22).22). It showed how the utilization of stored waste heat increases changed from case 1 to 4 to 22 (waste heat utilization fractions 54%, 71% and 89%, respectively). This was related to the temperature of the BTES. In case 1, the BTES temperature remained below 58 ◦C and cooled down to 39 ◦C at the end of the heating season (in April). In case 4, more heat was charged to the ground, raising the peak temperature to 65 ◦C, while the minimum temperature remained at 40 ◦C. The increased charging and discharging potentials were caused by the doubling of the number of boreholes, which directly relates to heat transfer power. In case 22, the size of the BTES was doubled compared with the other two cases. Thus, more heat could be charged into the ground. In this case, the peak temperature rose Buildings 2020, 10, 205 14 of 19

still higher to 67 ◦C, while the minimum temperature also went up to 49 ◦C. A larger share of district heating could be displaced because the BTES remained at higher temperatures. In case 22, the energy injected during the ﬁrst month of charging was more than three times the peak monthly demand in winter. This could be interpreted as there being signiﬁcant overcapacity in the BTES, but it was mainly becauseBuildings 2020 the, system 10, x was running constantly on full power during the charging period. 14 of 19

FigureFigure 14.14. MonthlyMonthly energyenergy ﬂowsflows andand averageaverage BTESBTES temperaturetemperature in in the the best best cases. cases.

InIt showed all cases, how the the charging utilization of the of BTESstored slowed waste heat down increases as summer changed progressed, from case because 1 to 4 to the 22 rising(waste temperatureheat utilization of thefractions BTES 54%, reduced 71% theand useful 89%, respecti fractionvely). of waste This heatwas related that could to the be temperature stored. Injecting of the moreBTES. thermal In case energy1, the BTES to the temperature BTES got harder remained as the below temperature 58 °C and di cooledﬀerence down between to 39 the °C storageat the end and of heatthe heating source diminished. season (in April). The monthly In case discharge 4, more levelsheat was showed charged how to the the BTES ground, was drained raising of the energy. peak Thetemperature stored energy to 65 levels °C, while at the the start minimum of the year temperat were aure result remained of the charging at 40 °C. done The during increased the previouscharging year.and discharging In each case, potentials the heat dischargedwere caused from by the the doub BTESling got of lower the number each month of boreholes, going from which December directly torelates April. to Thisheat wastransfer because power. of theIn case lower 22, temperature the size of the level BTES in was the BTES,doubled which compared limited with the the amount other oftwo useful cases. heat Thus, available. more heat However, could be atcharged the end into of th thee ground. year (September–December), In this case, the peak temperature the discharged rose energystill higher rose, to even 67 °C, though while BTES the minimum temperature temperature levels were also going went down. up to This49 °C. was A larger due to share the increasing of district heatingheating demand.could be Higherdisplaced demand because resulted the BTES in higherremained discharge at higher of temperatures. energy. However, In case even 22, in the the energy early discharginginjected during period, the thefirst relative month dischargeof charging rate, was with mo respectre than tothree the times demand, the waspeak going monthly down. demand in winter.The This heat could provided be interpreted from the storage as there replaced being 2760, significant 4220 and overcapacity 5720 MWh in of the conventional BTES, but district it was heatingmainly inbecause cases 1, the 4 and system 22, respectively. was running This constantly was on topon full of the power energy during provided the charging directly byperiod. waste heat in theIn summer. all cases, Assuming the charging the Finnishof the BTES national slowed average down for as district summer heating progressed, emissions because and thatthe rising heat recoveredtemperature from of wastethe BTES incineration reduced the was useful emission-free, fraction of the waste BTES heat system that reducedcould be annual stored. CO Injecting2 emissions more bythermal 536, 819 energy and 1109to the t-CO BTES2 in got cases harder 1, 4 as and the 22, temp respectively.erature difference This corresponded between the to storage 42%, 64%and andheat 86%source reduction diminished. in emissions The monthly in cases discharge 1, 4 and levels 22,respectively, showed how compared the BTES was with drained the case of using energy. waste The incinerationstored energy heat levels only at in the the start summer of the months. year were a result of the charging done during the previous year.Figure In each 15 case, displays the heat the hourlydischarged temperature from the development BTES got lower in theeach BTES month for going case 4. from December to April. This was because of the lower temperature level in the BTES, which limited the amount of useful heat available. However, at the end of the year (September–December), the discharged energy rose, even though BTES temperature levels were going down. This was due to the increasing heating demand. Higher demand resulted in higher discharge of energy. However, even in the early discharging period, the relative discharge rate, with respect to the demand, was going down. The heat provided from the storage replaced 2760, 4220 and 5720 MWh of conventional district heating in cases 1, 4 and 22, respectively. This was on top of the energy provided directly by waste heat in the summer. Assuming the Finnish national average for district heating emissions and that heat recovered from waste incineration was emission-free, the BTES system reduced annual CO2 Buildings 2020, 10, x 15 of 19

emissions by 536, 819 and 1109 t-CO2 in cases 1, 4 and 22, respectively. This corresponded to 42%, 64% and 86% reduction in emissions in cases 1, 4 and 22, respectively, compared with the case using waste incineration heat only in the summer months. Buildings 2020, 10, 205 15 of 19 Figure 15 displays the hourly temperature development in the BTES for case 4.

FigureFigure 15. 15.Hourly Hourly temperatures temperatures in in the the storage storage and and district district heating heating (DH) (DH) system system during during ﬁve five years years of of operationoperation in in case case 4. 4. The The ﬁgure figure shows shows the the average average temperature temperature in in the the storage storage volume volume as as well well as as the the inletinlet and and outlet outlet temperatures. temperatures.

ItIt shows shows the the average average temperature temperature of of the the whole whole storage storage volume volume as as well well as as the the inlet inlet and and outlet outlet temperaturestemperatures of of the the charging charging/discharging/discharging flow. flow. The The increase increase in in temperature temperature during during the the first first charging charging periodperiod was was large large and and the the BTES BTES almost almost reached reached the the equilibrium equilibrium temperatures temperatures duringduring thethe secondsecond year.year. AtAt the the startstart ofof each charging charging period period (excluding (excluding the the first first one), one), the the temperature temperature difference difference between between inlet inletand andoutlet outlet was wasclose close to 20 to °C, 20 but◦C, as but the as BTES the BTES heated heated up, the up, temperature the temperature difference difference lowered lowered to 6 to°C. 6Similarly,◦C. Similarly, at the at thestart start of ofthe the discharging discharging peri period,od, the the BTES BTES was was at maximal temperaturetemperature andand the the inletinlet/outlet/outlet di differencefference was was about about 35 35◦ C,°C, but but this this was was lowered lowered to to 15 15◦ C°C by by the the end end of ofthe theheating heatingseason. season. DuringDuring the the later later years years of of operation, operation, the the return return temperature temperature from from the the BTES BTES during during charging charging was was 55–7055–70◦ C.°C. This This was was still still a a useful useful temperature temperature level, level, which which meant meant that that the the output output from from one one seasonal seasonal energyenergy storage storage system system could could be be fed fed to to another another one. one. At At first, first, the the temperature temperature change change of of the the charging charging fluidfluid was was close close to to 20 20◦ C,°C, but but it it was was gradually gradually reduced reduced to to 6 6◦ C.°C. This This revealed revealed an an ine inefficiencyfficiency in in the the system,system, since since less less and and less less of of the the available available energy energy could could be be utilized. utilized. From From a a global global optimization optimization point point ofof view, view, it it would would bebe moremore usefuluseful toto heat several BTES systems systems to to lower lower temperature temperature levels levels instead instead of ofmaximizing maximizing the the utility utility of of a a single single system. system. The The result might be reducedreduced wastewaste heatheat utilizationutilization in in individualindividual communities, communities, but but a a rise rise in in total total e ffiefficiencyciency on on the the city city level. level.

4.4. Discussion Discussion

4.1.4.1. Toward Toward a a Net-Zero Net-Zero Energy Energy District District TheThe examined examined district district was was designed designed to to be be powered powered by by external external district district heating. heating. However, However, the the wastewaste heat heat provided provided by by the the incinerator incinerator plant plant was was stored stored in in a a local local storage storage system, system, reducing reducing reliance reliance onon the the gridgrid duringduring heatingheating season. Strictly Strictly speaking speaking,, the the district district is isnot not gene generatingrating its itsown own energy, energy, but butif the if the concept concept of ofnet-zero net-zero energy energy district district is is expa expandednded to to cover cover the the whole city, thethe communitycommunity asas a a wholewhole is is moved moved towardtoward net-zeronet-zero energy when previous previouslyly wasted wasted heat heat is is utilized. utilized. Looking Looking only only at the at theindividual individual district district likely likely results results in in suboptimal suboptimal system system performance performance because because of thethe highhigh returnreturn temperatures.temperatures. To To properly properly use use the the available available incineration incineration heat, heat, the the return return ﬂow flow from from one one BTES BTES system system shouldshould be be redirected redirected to to another, another, to to create create a city-wide a city-wide charging charging loop. loop. The The optimal optimal temperature temperature levels levels in suchin such aninterconnected an interconnected system system would would be worth be worth further further study. study. As shown in Figure 15, the temperature of the BTES remained above 40 ◦C at the end of the discharging period, meaning that there was still a lot of energy stored in the ground. With heat pumps, the total energy extraction could be doubled to bring the district close to zero carbon heating Buildings 2020, 10, 205 16 of 19

(depending on the type of electricity used). Such a district could completely stop utilizing conventional district heating, but to access the waste heat, a DH connection must be maintained regardless. Thus, it may be sensible to use the connection instead of investing in an overlapping heat pump system.

4.2. Implications and Potential In Finland, only 2.5% of municipal waste goes to landfills [21]. Most of the municipal waste is incinerated for energy. This means that adding to the total waste heat generation is not feasible. Thus, any increased energy use of waste should result from improved utilization rate of the energy generated during low demand. Here, the BTES system shows its importance. District heating in Finland is produced mainly by coal, peat and wood burning, so any shifting of excess waste heat to the heating period has a great potential for emission reduction. In the European Union (EU-27), 52 Mt of municipal waste is landfilled and 58 Mt is incinerated [22]. The final space heating demand in Europe is estimated to be over 300 TWh, of which the vast majority is produced by natural gas [23]. The share of district heating could be increased significantly, which leaves room for increased use of incineration heat. Most of Europe has increased heating demand in winter, which opens up the potential for storing the summertime excess heat using borehole thermal energy storage. If the heat storage concept were to be utilized to the maximum potential in as many communities as possible, any individual community could only utilize a fraction of the available excess heat. This would reduce the maximum temperatures possible to reach in the storage systems and create the need for heat pumps to provide more heat. However, the temperature of the ground would still be significantly elevated compared to the natural state, which would improve the coefficient of performance (COP) of the heat pumps. The waste heat could also be used to regenerate the ground to prevent extensive cooling in a scenario with high penetration of ground heat utilization. In such a regenerating role, the waste heat could be distributed widely with little consideration of the temperature. In a modern floor heating system, the heat distribution temperature is below 40 ◦C, while the DHW temperature requirement remains at 55 ◦C, to avoid Legionella. If DHW demands were met by building-side heat pumps, the district heating temperature could be lowered. This would reduce losses both in the DH distribution and the seasonal storage. Storage size might also be reduced by draining the stored heat more completely using heat pumps. Of course, the addition of heat pumps would entail costs that would need to be balanced with the obtained benefits. The LCOH of heat generated by the BTES was 10–23 €/MWh, which can be compared to the pre-tax average price of district heating in Finland, 66.5 €/MWh [24]. This leaves plenty of room for profits and payments to the waste heat generator.

4.3. Weaknesses and Reliability The series connection between the BTES and DH resulted in a high return temperature for the DH flow. This may be a problem for the utility that provides district heating. Another study could present an alternative system, which utilizes a parallel connection, maximizing the cooling in the DH flow. No actual heating grid was modelled, just the boundary of the residential zone. In practice, there would be some heat losses in the local district heating grid. Heat losses in district heating grids have been estimated to be 8–10% [25], but in a low temperature grid in a cold climate they could be around 5% [26]. Electricity consumption of water pumping was also disregarded. Measurements from a real BTES facility revealed that the electricity consumption of pumping was less than 0.4% of the heat injected into the storage [8]. Thus, the lack of pumping energy should not have a significant influence on the operational cost benefits. A parametric run does not provide optimal results because many potential system configurations are not tested. Another study could use multi-objective optimization, such as with a genetic algorithm, to further improve the cost-effectiveness of the incineration waste heat storage systems. Buildings 2020, 10, 205 17 of 19

5. Conclusions This study analyzed the potential of a seasonal borehole thermal energy storage system for increasing the utilization of waste heat in a residential community. Heat generated by waste incineration in summer was stored in the ground to preheat district heating flows during the heating season. The main finding was that the storage of waste heat turned out to be a promising low-cost measure to reduce district heating consumption and emissions of heating. The median storage efficiency was 61% in the 36 examined cases, even while the temperature of the storage volume exceeded 60 ◦C in many cases. This showed that relatively high temperature heat can be stored for long periods of time using large enough storage volumes. The many different simulated storage configurations revealed some guidelines for BTES design. Compared with only using waste heat from incineration in summer, the use of the BTES increased waste heat use by 78–325% in the 36 examined cases. In the three most cost-effective cases, the levelized cost of heat was 10.5, 12.9 and 18.9 €/MWh for waste heat utilization factors of 54%, 71% and 89%, respectively. Most of the heat required by the community could be provided by the incineration plant. The cost of heat compared to average district heating prices was low enough to be of interest to utilities. A more detailed study considering all the integration costs should be performed. A BTES system in Finland could be heated up to nearly the final operational temperature level in three years. The density of boreholes in the BTES was essential in determining the energy provided by the storage system. More boreholes meant more heating power and higher temperatures, which directly influenced how much useful heat could be recovered during the heating season. All the best BTES configurations utilized a wide and shallow storage system, which creates a stronger radial temperature gradient and reduces heat losses. When the number of boreholes was increased, it became useful to connect more boreholes in series, to increase the output temperature of the BTES at the cost of a lower flow rate. In many cases, the return temperature from the storage was high, which reveals a potential for further utilization of the same waste heat stream in other districts. The optimization of a network of storage systems to best utilize a singular waste heat source on the city level would be a good avenue for further study. In the best three cases, the emissions of district heating in the community could be reduced by 536, 819 and 1109 t-CO2/a through the use of the BTES, in addition to the direct use of the waste heat in summer. Operators of waste incineration plants should consider building seasonal thermal energy storage systems to communities within reach of heat-generating incineration plants.

Author Contributions: Conceptualization, J.H.; methodology, J.H.; software, J.H.; validation, J.H.; formal analysis, J.H.; investigation, J.H.; resources, J.H. and R.K.; data curation, J.H.; writing—original draft preparation, J.H.; writing—review and editing, J.H. and R.K.; visualization, J.H.; supervision, R.K.; project administration, R.K.; funding acquisition, R.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Academy of Finland under the “Optimal transformation pathway towards the 2050 low carbon target: integrated building, grids and national energy system for the case of Finland”, grant number 309064. Acknowledgments: The authors thank Vaasan Sähkö for providing the idea and basic data for the study. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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